Subproject 2.1

Physically-based parameterization of air- and water-side controlled transfer of volatile species across the ocean interface using combined laboratory and field experiments

Even after almost thirty years of intensive research on air-water gas transfer (the first International Symposium on Gas Transfer at Water Surfaces took place in 1983 at Cornell University), there is still no satisfactory physical-based model of gas transfer available. The newest review papers of Wanninkhof et al. [2009] and Jähne [2012] and the 6th International Symposium on “Gas Transfer at Water Surfaces” in Kyoto 2010 [Komori et al., 2011] clearly show the partially lacking experimental data and the still missing full understanding of the underlying processes. Consequently, still only semi-empirical relationships between the gas transfer velocity and the wind speed are available [Liss & Merlivat 1982, Wanninkhof 1992, and many others]. Without a physically-based model of the gas transfer process, it will, however, not be possible to reduce the current large uncertainties in the gas transfer rate with wind speed and to model the coupled ocean/atmosphere system adequately.

It is the goal of the third phase of this project to synthesize the laboratory and field results on air-water gas transfer obtained in the first two phases into a new physically-based parameterization. It should replace the old Wanninkhof and Liss-Merlivat type of wind-speed only parameterization of the air-sea gas transfer velocity. Given the more detailed investigation and new insight gained into the mechanisms of air-water gas transfer, this parameterization will include more parameters than the wind speed. Moreover, it will include both air- and water side control of the gas transfer in a single model, so that it is also possible to predict the gas transfer rates of environmentally important tracers with moderate solubility.

The synthesis of the laboratory and field results on air-water gas transfer obtained in the first two phases into a new physically-based parameterization of the air-water gas transfer velocity requires a number of steps: 1) Evaluation of all data from the first two phases of the SOPRAN project. 2) Establishment of a database containing all available field and laboratory after a critical review. The new models will be tested with a combination of existing data and the new SOPRAN data. 3) Establishment of a data base of critically evaluated physico-chemical properties of environmental importance (especially containing all those investigated by the various research groups in the first two phases of the SOPAN project). The most important parameters are the solubility of the tracer in water and the diffusion coefficients in air and water. For many tracers, solubility and diffusion coefficients are not very well established. It is planned to publish this data collection in a handbook. 4) Evaluation of the results from the Peru Meteor Cruise. This is the final field experiment for our group. It will be possible to make measurements of the gas transfer velocity with active thermography and small-scale wave imaging (also for characterization of water surface contamination by surface films) together with measurements of various VOCs in air and water (J. Williams), and studies of N2O gas exchange (H. Bange). The small-scale optical wave measurement system has been developed in cooperation with Lamont Doherty Earth Observatory, Columbia University, NY. The PhD student in our group doing this work (Daniel Kiefhaber), is funded by a separate DFG project. 5) Establishment and continuous refinement of a physically-based model for the air-sea gas transfer velocity with a detailed sensitivity and error analysis using the data base of gas exchange results (see 2.)

Fig.1: The Heidelberg Aeolotron, a large annular air-sea interaction facility.

6) Final verification of the new model with experimental conditions, where data are not yet available, with measurements in the Aeolotron together with the group of J. Williams, MPI Mainz, A. Engel, GEOMAR, M. van Pinxteren, TROPOS. External groups include O. Wurl, Oldenburg University, and two groups from England. Focus will be on the first air-sea gas exchange measurements with seawater in the Aeolotron including the effects of natural surface micro layer and bubbles. In addition aerosols will be sampled with different techniques to investigate, to which extend organic material from the surface micro layer will be carried into aerosol. The experiment will take place in November 2014. On September 23, twenty tons of Atlantic seawater arrived in Heidelberg.